Benzo-crown
ether-functionalized silica (BCES) was prepared by
the combination of the sol–gel process and the Schiff base
reaction, characterized by Fourier transmission infrared spectroscopy,
scanning electron microscopy, and N<sub>2</sub> adsorption–desorption,
and used for the selective adsorption of Ca<sup>2+</sup> ions. The
adsorption amounts of the BCES for Ca<sup>2+</sup> ions were independent
of pH in the range of 5–10. Adsorption equilibrium was performed
within 60 min. The adsorption capacity of the BCES for Ca<sup>2+</sup> ions was found to be 40.6 mg g<sup>–1</sup> at 298 K. The
high selectivity of the BCES for Ca<sup>2+</sup> ions with respect
to the other alkali and alkaline earth metals was attributed to the
high affinity between Ca<sup>2+</sup> ions and crown ether. The adsorption/desorption
cycle of the BCES could be up to five times. Langmuir and pseudo-second-order
equations provided good correlation for the adsorption of Ca<sup>2+</sup> ions. The thermodynamics of Ca<sup>2+</sup> ions/BCES system indicates
spontaneous and endothermic nature.